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A small photosensitive chip implanted in the retina has made a huge difference to the vision of patients suffering from the degenerative eye disease retinitis pigmentosa, US researchers have shown. And it seems that an implant in one eye can actually improve vision in the other.

Dr. Ronald Schuchard, a leading researcher in the field, told delegates at the Vision 2005 conference in London that his team were at a loss to explain some of their results, particularly: improvements in vision in the non-implanted eye; improvements in areas of the retina that should not have been affected by the surgery; and improved colour perception, despite the fact that the implant is not capable of detecting or distinguishing colours. Vision 2005 is an annual conference that takes in all aspects of sight loss, including technologies and medical advances that could help people overcome blindness.

Schuchard's team implanted a 2mm diameter silicon chip, covered in photodiodes, in the superior temporal retinal area of one eye of each of ten volunteers. (For the non specialists, that is not right in the middle of the retina, but up and to one side.) The chip captures light that enters the eye, and generates an electrical signal that is transmitted to the overlaying neural cells of the retina.

Schuchard said: "We found that visual function improves not just in the area of the chip, but in the macular area too. We have no way of explaining this: there is no observable change in the macular area."

Patients also reported stretched contrast perception: from being able to see only a kind of greyish blur, one patient could see darker and lighter areas, post operation. Other patients reported restored or improved colour perception. One man said he was able to see blue for the first time in 20 years.

"We don't have good control ways of testing for these things," Schuchard told delegates. "We believe that the implant is not directly helping, but possibly causing a neurotropic effect. We believe it is to do with a recovery of photoreceptors that hadn't completely died off. Researchers are going after the mechanism, so guess what you'll be hearing about very soon."

The volunteers' vision before the implants ranged from bare light perception to being able to read fewer than ten letters on a sight chart at half a meter's distance. After the chip was implanted, the volunteers were re-tested and every single one showed a significant improvement: some patients could read 25 letters on the sight chart, post operation.

The improvement was best in the first six months, and sight does deteriorate again until about twenty months after the chip is implanted, although it is still significantly better than pre-operative sight. ®

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